A global marine survey turned up just a tiny fraction of the expected amount of plastic debris, but scientists aren’t cheering. Now researchers have to figure out where the rest of the oceans’ plastic trash went.

Estimates put the amount of plastic entering the world’s oceans in the range of millions of tons. But a research group led by Andrés Cózar, a marine ecologist at the University of Cádiz in Spain, reports June 30 in the Proceedings of the National Academy of Sciences that between 7,000 and 35,000 tons of plastic debris floats on the sea surface. That would mean 99 percent of the plastic thought to be in the oceans is missing. Even still, on the high end that’s the equivalent of 10 half-liter water bottles in each square kilometer of ocean. Much of the plastic accumulates in major rotating currents called gyres, including the infamous Pacific garbage patch.

The plastic comes from a variety of sources, including trash floating down rivers or blown off garbage barges, fishing gear lost or abandoned at sea and tiny beads from cosmetics washed into sewers. When plastic reaches the ocean, the sun, waves and wind weaken large pieces, which break down into smaller chunks, Cózar says, until most plastic is in tiny fragments floating on the ocean surface.

Cózar’s team used data from five ships on global expeditions between 2009 and 2013 that collected more than 10,000 pieces of plastic in fine mesh nets. Because plastic floats, the researchers sampled only the top half-meter or so of ocean. Using the concentrations of plastic in their samples and in data collected by other researchers, along with wind and current patterns, the team estimated the global oceans’ total plastic load.

The largest numbers of plastic pieces were about two millimeters across. The abundance of pieces smaller than that dropped off quickly, however, contrary to their expectations.

The authors suggest a number of places that the missing plastic could have gone. The most likely explanations, Cózar and colleagues say, are that the plastic was broken down by microbes or eaten by fish and other animals. In other tests, the scientists found plastic in the stomachs of small filter feeders called lantern fish that eat prey about the same size as the missing plastic. These kinds of fish, Cózar says, are the main prey for commercial fish such as tuna and swordfish.

“I am not surprised by the discrepancy between this estimate and the amount of plastic produced annually,” which formed the basis for earlier estimates of plastic trash in the ocean, says Kara Lavender Law, an oceanographer at the Sea Education Association in Woods Hole, Mass. “We know next to nothing about timescales of biodegradation.”

She adds that there are plastic reservoirs the group didn’t measure, such as large debris including commercial fishing gear or flotsam from the 2011 Japanese tsunami and other disasters. But because there is so little information about ocean plastics, it’s hard to know what fraction of total ocean plastic these reservoirs might represent. That’s one reason she praises the paper for providing data about plastic in the oceans of the Southern Hemisphere, where she says there are “very, very few measurements.”

Both Lavender Law and Cózar agree that plastic pollution in the ocean represents an urgent problem because of the risks it poses to wildlife. “Regardless of its ultimate fate, we should be working now to reduce the flow of plastic into the ocean,” Lavender Law says.

Editor's Note: This story was updated July 16, 2014, to correct the dates of the expeditions and the number of plastic pieces collected.